/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode res = null;
        ListNode ptr = null;
        int carry = 0;
        while (l1 != null || l2 != null) {
            int n1 = l1 != null ? l1.val: 0;
            int n2 = l2 != null ? l2.val: 0;
            int sum = n1 + n2 + carry;
            if (res == null) {
                res = ptr = new ListNode(sum % 10);
            } else {
                ptr.next = new ListNode(sum % 10);
                ptr = ptr.next;
            }
            carry = sum / 10;
            if (l1 != null) {
                l1 = l1.next;
            }
            if (l2 != null) {
                l2 = l2.next;
            }
        } 
        if (carry > 0) {
            ptr.next = new ListNode(carry);
        }
        return res;
    }
}